The most common type of waveguide propagates signals in only one specific electromagnetic field pattern or mode, out of an infinite number of possible modes. Single-mode operation occurs because the waveguide is designed so that signals are in a frequency band which is sufficiently low that only the mode with the lowest "cutoff frequency" can exist and no other mode can propagate. If other modes were allowed to propagate, signal energy could couple into and out of various modes substantially distorting the signal. Such "conventional waveguide" is compact and easy to design, model and use. Unfortunately, maintaining only the lowest-cutoff mode in a given frequency band requires restriction of the waveguide cross section and this, in turn, restricts power carrying capacity and limits the lowest achievable signal attenuation. As a result, design of some systems requiring microwave or millimeter wave signal transmission with high power or very low loss may be difficult or impractical.
An alternative type of waveguide is generally called "overmoded" in which a higher order mode is used, i.e. a mode which does not have the lowest cutoff frequency. Because other (unwanted) modes are also capable of existing as well as the desired transmission mode, this type of waveguide must feature internal structures which suppress the unwanted modes. Because internal structure, rather than restriction of cross section dimensions, is the basis for suppressing all but the desired mode, overmoded waveguide cross section can, in principle, be made arbitrarily large for a corresponding increase in power capacity and decrease in signal attenuation. Unfortunately, this type of waveguide, with unwanted mode suppression, is difficult to model and design, and its cross-sectional dimensions may not be amenable to compactness without significant design optimization. A computer-aided method for designing such optimized overmoded waveguide is described in copending and commonly assigned application, Ser. No. 310,193 filed Feb. 13, 1989 which issued as U.S. Pat. No. 5,046,016 on Sep. 3, 1991.
Historically, the more successful type of overmoded waveguide supports the circular TE.sub.01 mode, e.g. see H. E. Rowe and W. D. Warters, "Transmission in Multimode Waveguide with Random Imperfections", Bell System Technical Journal, Vol. 41, No. 3, pp. 1031-1070, May 1962. Such waveguide uses either a dielectric lining or dielectric sheathed helix of insulated wire inside the circular cross section waveguide for suppression and decoupling of unwanted modes, e.g. see A. E. Karbowiak "Trunk Waveguide Communication", Chapmen and Hall Ltd. 1965. Both versions of overmoded TE.sub.01 waveguide were originally developed and tested for millimeter band (60-100 GHz) trunk line telecommunications between cities. Application of overmoded waveguide technology for high power and/or low loss transmission in microwave or millimeter wave radio communications and radar has also been suggested and developed to a limited degree, e.g. see R. M. Collins "Practical Aspects of High Power Circular Waveguide Systems" NEREM Record, Session 24, pp 182-183,(1962).
Circular TE.sub.01 mode waveguide systems generally feature mode suppression, either distributed filtering along the transmission length or at discrete intervals. As a result, the relatively low power is coupled into unwanted modes by waveguide imperfections, bends, and transitions, and unwanted mode energy that does arise is converted to heat. One apparent feature of mode suppression filtering is that components in overmoded waveguide may be matched to the desired TE.sub.01 mode at terminations and transitions; however, the undesired modes are generally not. See for example, A. P. King and E. A. Marcatili, "Transmission Loss due to Resonances of Loosely Coupled Modes in a Multimode System", Bell System Technical Journal, Vol. 35, pp. 899-906 (1956). The resulting reflections due to high VSWR for these unwanted modes can lead to trapped resonances and inefficient mode suppression. This can be especially of concern for high power capacity systems in which although only a small percentage of energy is coupled into unwanted modes, appreciable RF energy is built up without proper purging from the system.
The prior art includes various structure for filtering or suppressing unwanted modes. For example, U.S. Pat. No. 2,760,171 to King discloses a mode filter consisting of a circular metallic waveguide filled with several pieces of dielectric material, each with a pie-shaped cross-section. Between each pair of these pie-shaped spacers is placed a resistive card. These cards must be tapered to minimize spurious mode reflectivity. This prior art device does not provide for an impedance match for the TE.sub.01 mode and there is some residual TE.sub.01 mode reflectivity because of the interface between the dielectric spacers and the rest of the waveguide which is air-filled. Further, the filled cross-section of this patented structure is not compatible with very low loss and high power operation contemplated by the present invention, due to the edges of the resistive cards, potential dielectric strength problems, and dielectric losses introduced thereby. Moreover, as will be explained, the present invention has the distinct advantage of locating the structural variations outside the region of the TE.sub.01 mode propagation.
The Albersheim U.S. Pat. No. 2,779,006 relates to TE.sub.01 mode transmission through curved waveguide and it utilizes transverse slots in the waveguide bend to minimize generation of the spurious modes. Although this patent mentions the problem of providing an impedance match between the slotted and unslotted sections of waveguide, it does not describe how one might accomplish this.
The Clogston U.S. Pat. No. 2,948,870 teaches the placement of small ferromagnetic discs at the axis of a circular waveguide for mode suppression. The electromagnetic properties of these discs are controlled by an externally generated D.C. field. This device does include lossy dielectric tapers for the purpose of an impedance match, apparently for both the TE.sub.01 and the undesired modes. Since these tapers do have lossy components, they do not maximize TE.sub.01 transmission. Furthermore, as noted, the transitions and the mode-suppressing discs are located in the center of the waveguide and are supported by polystyrene spiders. Each spider consists of a hub through which runs a cylindrical member of dielectric material containing the discs, the hub being supported in the center of the waveguide by spokes or arms radiating from the hub to the waveguide's interior wall. These spiders cause additional TE.sub.01 reflectivity and there are no tapered structures located in this device to minimize this additional reflectivity. In contrast, in accordance with the present invention, all of the mode suppressions in the proposed structure are located on the periphery of the waveguide (e.g. outside the helix wall supporting the TE.sub.01 mode), no spiders are needed, and all of the varying physical and electrical characteristics are introduced in a gradual manner along the length of the waveguide.
In the device taught in the Nakahara et al U.S. Pat. No. 3,601,720, the waveguide lining has a varying dielectric constant so that in some parts of the waveguide the TE.sub.01 mode is damped while in other parts the TE.sub.12 mode is damped. In contrast to the present invention, there is no provision in this prior art reference for a varying thickness other than as a taper in one configuration. However, this patent does not address the problem of purging all residual unwanted mode loss and matching to an end transition, as accomplished by the present invention.